1. Field of the Invention
The present invention relates to a pigment dispersion and an ink composition, curable composition and a curable ink composition making use of the same. More particularly, the invention relates to a pigment dispersion having satisfactory pigment dispersibility and excellent storage stability under high temperature conditions, an ink composition, curable composition and a curable ink composition making use of the dispersion.
2. Related Art
Image recording methods of forming an image on a medium to be recorded such as paper or a plastic sheet, based on image data signals, involve electrophotographic systems, sublimation type or melt type heat transfer systems, inkjet systems and the like. Among them, inkjet systems can be operated with inexpensive recording apparatuses, and perform image formation directly on a medium to be recorded, by jetting an ink composition (hereinafter, also simply referred to as “ink”) only on the image areas where needed. Thus, the inkjet systems can utilize ink with high efficiency at low running costs. Furthermore, the inkjet systems are excellent from the viewpoint of allowing droplet striking of very fine liquid droplets, and thereby enabling recording of highly colorful images.
According to the image recording methods involving inkjet systems, printing is possible not only on ordinary paper but also on non-absorptive media to be recorded, such as a plastic sheet and a metal plate. However, increasing the speed and image quality upon printing is becoming an important goal, and the inkjet systems are characterized in nature such that the time required for drying and curing of liquid droplets after printing greatly affects the sharpness of images.
One of the inkjet systems is a recording system making use of a curable ink composition that is curable upon irradiation with active energy rays. According to this method, ink liquid droplets can be cured by irradiating the droplets with active energy rays immediately after printing, and thereby images with sharp lines can be formed.
Such a curable ink composition is required to have high pigment dispersibility, for the formation of high precision images having excellent color-developing properties. Furthermore, it is important to control the viscosity or surface tension of the ink, in order to jet out fine ink droplets from the fine pores of an ink jetting head in the inkjet systems. In order to impart vivid brightness and high coloring power to an ink composition, micronization of pigment is essential. Particularly, in those inks used for inkjet recording, since the ink droplets that are jetted out exert great influence on the sharpness of images, the amount of liquid droplets being jetted is also reduced, and it is essential to use particles that are even finer than the thickness of the cured ink films formed as images. As such, if the pigment particles are micronized more and more so as to obtain high coloring power, dispersion of the microparticles becomes difficult, and aggregates are likely to be generated. There also occurs a problem that the addition of a dispersant that is used to enhance the dispersibility of pigment particles, causes an increase in the viscosity of the composition. The generation of pigment aggregates and an increase in the viscosity of ink composition both lower the ink jetting properties, and therefore, using an ink composition in which aggregation of pigment or viscosity increase has occurred, for inkjet recording, should be avoided.
In the case of using an ink composition for inkjet recording, the heat cycle properties must be excellent. The ink composition is held in a cartridge, and is heated during jetting, in order to lower the liquid viscosity. However, when jetting is ceased, since the temperature decreases during storage, the ink composition is subjected to repeated temperature changes involving heating and cooling. These temperature changes also exert adverse influence on the pigment dispersibility, so that there arises problems such as that the pigment dispersibility is lowered along with a lapse of time, and aggregation of pigment, viscosity increase or the like easily occurs.
In regard to a curable ink composition, the curability on objects to be printed and the film properties after curing also serve as important factors. In a curable ink composition, since the sharpness of images is secured by rapidly curing the composition by irradiation with active energy rays, a pigment dispersant that is prone to inhibit curing, a pigment dispersant containing a solvent that is a non-curable component, and a pigment dispersant that does not crosslink with the cured film and decreases the surface tackiness, lower the curability and become the causes of bleeding or a decrease in productivity, which is not preferable. When such compounds are incorporated, there occurs a problem that so-called blocking occurs, in which when printed matters are stored in a piled state, the printed surface of a printed matter and the surface of another printed matter that is superimposed with the printed surface (usually, the backside) stick together, or the contacting surfaces are mutually damaged by contamination.
Therefore, there is a demand for an ink composition which has sufficient fluidity during the storage before use, and maintains a micronized pigment in a stably dispersed state for a long time, and which has excellent curability such that the composition rapidly cures after being jetted to the surface of an object to be printed.
Various suggestions have been made on ink compositions containing a dispersant that efficiently disperses a pigment in the ink and has an effect of maintaining the dispersed state stably over a long time period.
For example, there have been proposed an ink composition containing, as a dispersant, a pigment derivative that enhances the affinity with pigments (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-119414 and 2004-18656); an ink composition containing a polymer having a basic group, as a dispersant to specific pigments such as phthalocyanines and quinacridones (see, for example, JP-A No. 2003-321628); an ink composition which contains a dispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer, and a specific monomer capable of dissolving the dispersant, but does not contain a volatile organic solvent (see, for example, JP-A No. 2004-131589); an ink composition containing, as a dispersant, a polyurethane having a neutralized salt such as carboxylate, phosphate, sulfonate or quaternary ammonium, in a pendant (see, for example, Japanese Patent Application National Publication (Laid-Open) No 2002-503746); a non-aqueous pigment-dispersed composition making use of a graft copolymer prepared using a nitrogen atom-containing, polymerizable vinyl macromonomer (see, for example, JP-A No. 2007-277506); and an ink composition containing, as a dispersant, a polyurethane that has a polymer containing an N-vinylamide, an N-vinyl lactam or a vinyl- or allyl-substituted nitrogen-containing heterocycle, and another polymer as graft chains (see, for example, Japanese Patent Application National Publication (Laid-Open) No. 2002-526254).
These ink compositions have pigments definitely finely dispersed therein so that the ink stability is higher than the conventional ink compositions. However, when these compositions were used as low viscosity ink compositions for the use in inkjet recording, the ink stability at high temperatures was not satisfactory; and when these compositions were applied to curable inks, the inks lacked curing sensitivity and still needed further improvement in blocking resistance.
Furthermore, it has been suggested that an ink composition containing an acrylic copolymer having a tertiary amino group and the organic chromophoric atomic group of a pigment, as a dispersant, results in an ink that is excellent in fluidity, clear imaging properties of the film, glossiness and coloring power (see, for example, Japanese Patent No. 2993088). However, the dispersant did not have a sufficient affinity with the components that are incorporated as ink media, such as polymerizable compounds, and still needed further improvement in stability.